AGK, along with the sphingosine (SPHK1, 2) and ceramide (CERK) kinases and the ceramide kinase like protein (CERKL), comprise the sphingolipid kinase family. AGK is unique among members of this family in that it is a mitochondrial protein. We discovered that mice lacking a functional AGK allele die early in embryogenesis due to failure to implant, which is also unlike SPHKs, CERK or CERKL where null mice are viable and fertile. However, the lipid substrate of phosphoryl transfer reaction catalyzed by AGK is uncertain. The research program we propose will discover that substrate/product and in doing so will define a lipid metabolic pathway that is most likely crucial to mitochondrial survival. Specifically, we will: (Aim 1) Generate matched pairs of cell cultures and tissues wherein AGK expression is markedly different and use mass spectrometry to characterize the lipidome of those cells and tissues so as to ultimately identify the reaction catalyzed by AGK and (Aim 2) characterize AGK function in mitochondria by determining its sub- organelle location, studying mitochondrial physiology in cells deficient in AGK and using conditional deletion of AGK alleles to determine the fate of cell lineages in the mouse. Our extensive experience studying lysophospholipid chemical biology including sphingosine kinases coupled with expertise in mitochondrial physiology will enable us to solve this problem. Minimally, the experiments proposed will reveal a new branch of sphingolipid metabolism. Maximally, we will define a new pathway that is integral to mitochondrial function.